In recent years, CIS-based thin film solar cells, which use Group I-III-VI2 compound semiconductors of chalcopyrite structures including Cu, In, Ga, Se, and S as the p-type light absorption layers, have become the focus of attention. This type of solar cell has a relatively low manufacturing cost, and further a large absorption coefficient in the wavelength range from visible light to near infrared light, so promises a high photoelectric conversion efficiency and is regarded as a leading candidate for the next generation of solar cells. As typical materials, there are Cu(In, Ga)Se2, Cu(In, Ga) (Se, S)2, CuInS2, etc.
A CIS-based thin film solar cell is comprised of a glass substrate on which a metal back surface electrode layer is formed, a p-type light absorption layer comprised of a I-III-VI2 group compound which is formed on top of that, and furthermore an n-type high resistance buffer layer and an n-type transparent conductive film window layer which are formed on top of that. In such a CIS-based thin film solar cell, it is reported that when using soda lime glass as the glass substrate, a high photoelectric conversion efficiency can be achieved.
This is believed to be because the Group Ia elements (in particular Na) which are contained in soda lime glass diffuse by heat treatment in the p-type light absorption layer in the process of forming that layer and promote crystal growth and have an effect on the carrier concentration. On the other hand, if the amount of Na which is introduced into the p-type light absorption layer is too great, the problem has also been pointed out that peeling from the back surface electrode layer easily occurs. Therefore, when manufacturing a CIS-based thin film solar cell, introduction of the optimum amount of Na into the p-type light absorption layer is extremely important in improving the photoelectric conversion efficiency.
The method has been proposed of providing an alkali diffusion prevention layer of a thickness of 20 to 100 nm made of aluminum oxide, titanium nitride, silicon dioxide, etc. between the soda lime glass substrate and the back surface electrode layer so as to completely prevent the diffusion of Na etc. from the glass substrate to the inside of the p-type light absorption layer and, on the other hand, when forming the p-type light absorption layer, adding Na or another alkali metal from the outside so as to attempt to accurately control the content of alkali metal in the p-type light absorption layer (see PLT 1).
Further, the method has also been proposed of, instead of adding Na etc. to the p-type light absorption layer, introducing a Group Ia element, for example, Na, which is contained in the soda lime glass substrate into the p-type light absorption layer in a suitable amount during the process of forming the layer by means of providing an alkali control layer of a thickness of 20 nm to 50 nm made of silica etc. between the soda lime glass substrate and the back surface electrode layer (see PLT 2).
On the other hand, to improve the photoelectric conversion efficiency of a CIS-based thin film solar cell, it is necessary, it is pointed out, to make the film forming temperature when forming the p-type light absorption layer, that is, the selenization and sulfurization temperature, a high temperature. By performing the film forming process at a high temperature, the quality of the p-type light absorption layer is improved and, as a result, the photoelectric conversion efficiency is also improved. Soda lime glass has a relatively low strain point, therefore, if forming the p-type light absorption layer at a high film forming temperature, for example, 550° C. or more, so as to further raise the photoelectric conversion efficiency, the glass substrate will deform, so the film forming temperature cannot be raised. As opposed to this, PLT 3 discloses to use high strain point glass as the substrate of the CIS-based thin film solar cell so as to suppress deformation of the glass substrate due to the heat history and strain due to the difference in thermal expansion coefficients between the substrate and the CIS-based semiconductor layer.
Here, the inventors ran experiments in which they used high strain point glass as the glass substrate and raised the temperature of the sulfurization or selenization to a high temperature. As a result, strain of the substrate of the solar cell which was produced could be suppressed, but a high photoelectric conversion efficiency could not be achieved. This was due to the fact that high strain point glass is low Na glass, so diffusion of Na to the p-type light absorption layer was insufficient. Further, in a further experiment of the inventors using high strain point glass as the glass substrate, to sufficiently introduce Na into the p-type light absorption layer, a 30 nm to 50 nm alkali barrier layer which was disclosed in the above PLT's 1 and 2 was provided and Na was added to the p-type light absorption layer, but in this experiment as well, a solar cell which has a high photoelectric conversion efficiency could not be obtained.